Marco tried a new fruit-flavored beverage and thought it was awful. He was especially disappointed because he had liked the dried fruit snacks marketed under the same brand name. Now he wasn't sure he even wanted to buy the snacks he had liked before. This highlights a problem in branding known as:

Brand dilution.

Explanation:

Brand dilution is the mechanism through which a certain brand is weakened because of its overuse.Brands becomes less meaningful due to using products or items that are not good fit for brand  .They lose effectiveness and wort due to unsuccessful brand extension.

According to the question,brand dilution is the issue of the scenario as fruit flavored beverage is an unsuccessful brand extension of the same brand whose dried fruit snack is liked by Marco.This is making him doubtful about the snack also because of ineffective product.Thus,brand lost its power.